A known electrophotographic image forming apparatus includes a photosensitive drum, a charger, and a transfer roller. The charger charges a surface of the photosensitive drum, the charged surface of the photosensitive drum is irradiated with a laser beam, and an electrostatic latent image is formed on the surface of the photosensitive drum. Toner is applied to the electrostatic latent image, and the electrostatic latent image is developed into a toner image on the surface of the photosensitive drum. The toner image on the photosensitive drum is transferred to a recording medium such as a sheet of paper by a transfer roller, the transferred image is fixed by heat onto the recording medium by a fixing device, and thus the image is printed.
In addition, such an image forming apparatus is generally structured to unitize a photosensitive drum, a charger, a transfer roller and other parts into a removable cartridge for easy exchange of consumables.
The charger includes a discharge electrode that generates a corona discharge and a discharge induction electrode. The discharge electrode may be a wire. The discharge induction electrode is spaced from the discharge electrode. The charger is designed to charge the photosensitive drum by the corona discharge from the discharge electrode. The charger thus designed needs exchanging regularly because it deteriorates with use or mainly a discharging performance lowers due to adhesion of silica to the discharge electrode.
Japanese Laid-Open Patent Application No. 9-68848 discloses a structure that enables only a charger to be removed from a process cartridge.
The discharge electrode of the charger is subjected to application of higher voltage (or discharge bias) and deteriorates rapidly compared with the discharge induction electrode. Conversely, the discharge induction electrode of the charger is less prone to deteriorate and has a longer service life compared with the discharge electrode. However, the structure disclosed in Japanese Laid-Open Patent Application No. 9-68848 requires exchange of a whole charger. This means that the discharge induction electrode should be exchanged along with the discharge electrode even if it is still available, with the result that the discharge induction electrode is not used effectively.